1. Field of the Invention
The present invention relates, in general, to methods for producing high-temperature superconducting Josephson devices and, more particularly, to methods for producing step-edge type high-temperature superconducting Josephson devices using a chemical etching solution of phosphoric acid and sulfuric acid to form substrate steps, without the use of ion milling and sputtering processes, thereby easily producing the devices and making a large area of etched substrate.
2. Description of the Prior Art
Ever since a high-temperature superconductor was found, various types of superconducting Josephson devices have been produced. However, high-temperature superconductor manufactures have significant difficulty in producing good quality junction devices because their crystallization temperature is higher and their coherence length is shorter than metal-based superconductors, and because there is crystal anisotropy. To avoid this difficulty, a bridge type Josephson junction using a crystallographic grain boundary of a YBCO (YBaCuO.sub.y) thin film has been studied, but since it is virtually impossible to control the junction location, size and quality due to the properties of the material, this technique is not advantageous. Recently, all over the world, many attempts have been made to artificially manufacture weak link devices. There are three principal techniques: a step-edge process of forming the steps in a substrate to control the location at which a crystallographic grain boundary is formed, a bi-crystal process of making the crystal orientation be somewhat different, and a bi-epitaxial process of utilizing a buffer layer to rotate the crystal orientation at an angle of 45.degree..
Among the junctions described above, it is known that the step-edge junction is easily manufactured relative to the other junctions and therefore is being actively researched. This type of junction is shown in FIG. 1. As shown in this figure, sharp steps are formed on a usual substrate in such a way that the steps are angled by 45.degree. or more relative to the surface of the substrate, followed by growth of an epitaxial YBCO thin film on the steps, so as to produce a crystallographic grain boundary at each of the step-edge portions.
At present, most researchers mainly use an ion-milling process or a sputtering process for the manufacture of the step-edge junction in a substrate. The disadvantages of these techniques are that the apparatus used are expensive and mass production and reproducibility are not readily attained. these are obstacles preventing the practical use of the these techniques.
To overcome these problems, U.S. Pat. No. 5,196,395 discloses that an oxide substrate is etched by a chemical etchant, to form V-steps for production of a high-temperature superconducting device. In more detail, a high-temperature superconducting thin film is coated over the V-steps manufactured by a chemical etchant, to produce a crystallographic grain boundary at each of the step portions, which is utilized as a Josephson junction. A formation process for a Josephson junction in this patent consists broadly of the following steps. First, V-steps are formed on a substrate by acid etching with a metal film serving as a mask. A high temperature superconductor (T1 type) is coated onto the V-steps and then a typical patterning process is used. However, as shown in FIG. 3A when the chemical etchant is made of phosphoric acid and sulfuric acid (4:1) as suggested in the supra patent, the surface condition of the substrate after etching is poor and thus there is a large possibility that a weak link might arise all over the substrate. In general, a substrate having a poor surface condition, e.g. many irregularities, is unsuitable for formation of a junction and it is virtually impossible to control the location of a junction and the junction properties with such a substrate. If a weak link is formed at an undesired position, the lithography cannot be carried out well, which is therefore a very serious problem. Consequently, it is fundamentally impossible to make a SQUID device using a weak link junction.